1. Field of the Invention
This invention relates to the field of processes of distillation and, more particularly, to processes for the recovery by distillation of anhydrous ethanol from a dilute aqueous feedstock.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline--10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to provide a dilute solution of ethanol ("beer") and the recovery of anhydrous or concentrated ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials.
To date, however, relatively little concern has been given to the energy requirements for manufacturing ethanol, especially with regard to the ultimate distillation operation which is the most energy intensive procedure in the ethanol production sequence whether the ethanol be derived from a petroleum or vegetative source.
The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources. The most common subject operation employs cane sugar in a fermentation-distillation operation which conveniently utilizes the bagasse by-product as a fuel source.
Processes for the azeotropic distillation of a dilute ethanol feed to provide absolute alcohol are well known (viz., U.S. Pat. Nos. 1,583,314; 1,486,717; 1,586,732; 1,670,053; 1,761,779; 1,763,722; 1,830,469; 1,873,005; 1,935,529; 2,050,513; 2,386,058; 2,640,017; 2,695,867; 3,404,186; and 3,960,672). In a typical anhydrous distillation process, a concentrated ethanol stream is combined with benzene (or other azeotrope-forming liquid), and the mixture is heated in a distillation column to provide a ternary vapor mixture containing ethanol, benzene and water at the top of the column, a binary mixture of ethanol and benzene in the middle of the column and absolute ethanol at the bottom of the column. Part of the vapors at the head of the column are condensed and the condensate is returned to the top of the distillation column as reflux. The remaining part of the vapors are condensed and separated in a decanter or like apparatus into a benzene-rich upper layer which is returned to the distillation column and an aqueous ethanol-rich lower layer from which residual benzene is removed and recycled. When the source of ethanol for this and similar anhydrous distillation processes contains one or more low boiling compounds such as ethyl acetate in amounts which would tend to interfere with good separation of the benzene-rich and aqueous ethanol-rich layers in the decanter, it is necessary that such compounds be removed from the feed prior to anhydrous distillation. Commonly assigned, copending U.S. patent application Ser. No. 043,189, filed May 29, 1979, describes a method for the anhydrous distillation of a dilute aqueous ethanol feed in which low boiling components are removed from the feed in a heads stripping column prior to concentration of the ethanol in a rectifying column. Desirable as this procedure may be, the operation of the heads stripping column in application Ser. No. 043,189 accounts for a significant amount of the overall energy requirements of the system.